During the production of highly doped semiconductor layers that are at the same time of high crystalline quality in semiconductor chips, in the case of many materials, particularly in the case of wide-band semiconductors, a codoping with a second material is necessary alongside the actual doping. By way of example, an electron acceptor material is introduced in the case of a high p-type doping sought to produce an increased charge carrier concentration, that is to say an increased hole concentration. At the same time to counteract a degradation of the crystal quality, an electron donor material is additionally introduced as codoping, as a result of which, however, the electrical neutrality of the crystal is at least partly reestablished. The codoping can thus be undesirable but required by the production method. In the case of layers that are p-doped and simultaneously n-codoped in this way in accordance with the example mentioned, the codoping results, however, in only a low p-type doping or even an intrinsic charge carrier concentration or even an n-type doping. For a high hole concentration for the high p-type conductivity sought in the example mentioned, the compensating effect of the codopant has to be cancelled again, which is referred to as so-called activation of the electrical conductivity, the p-type conductivity in the example given, or as activation of the dopant.
The electrical activation of such codoped semiconductor materials is usually achieved by the activation in the form of a purely thermal annealing step. This requires the codopant to be more readily volatile than the dopant and for the codopant to be able to be driven out from the doped semiconductor layer to a certain degree or completely, that is to say for example from 0.001% to 100%, by the thermal annealing step. This method is necessary for example for the activation of the p-type side of GaN-based light-emitting diodes (LEDs). For this activation there exist established methods for example particularly on the basis of so-called RTP (“rapid thermal processing”) processes under specific atmospheres. Conventional activation processes take place at a high temperature of 700 to 1000° C. in the wafer assemblage in the form of RTP processes or else at a lower temperature at 500 to 600° C. in the wafer assemblage in the tube furnace with a comparatively significantly longer duration and a different gas mixture.
The known methods function only inadequately, however, if for any reason the diffusion of the codopant from the doped semiconductor material is prevented, as is the case for example in so-called buried p-doped layers. In this case, there are significant differences in the achievable degree of activation for codoped p-type layers which are exposed, that is to say which lie near a surface of the crystal, and for which the conventional methods described function, and for p-doped layers which are buried below one or more layers, in particular n-doped layers. The latter can be activated only slightly or not at all by the known activation methods. The measurable operating voltage of components, such as of LEDs, for example, is thereby significantly increased.
It could therefore be helpful to provide methods for producing an optoelectronic semiconductor chip which has at least one doped functional layer. It could also be helpful to provide an optoelectronic semiconductor chip.